The present invention relates to a process for making carbonless copy paper coatings containing melamine-formaldehyde (hereinafter referred to as "MF") microcapsules and to carbonless copy papers coated therewith. More particularly, the present invention relates to a process of preparing MF microcapsules in a medium of water containing high concentrations of water soluble or water miscible organic solvents.
In the manufacture of pressure-sensitive recording papers, better known as carbonless copy papers, a layer of pressure-rupturable microcapsules containing a solution of colorless dyestuff precursor is normally coated on the back side of the front sheet of paper of a carbonless copy paper set. This coated back side is known as the CB coating. In order to develop an image or copy, the CB coating must-be mated with a paper containing a coating of a suitable color developer, also known as dyestuff acceptor, on its front. This coated front side is called the CF coating. The color developer is a material, usually acidic, capable of forming the color of the dyestuff by reaction with the dyestuff precursor.
Marking of the pressure-sensitive recording papers is effected by rupturing the capsules in the CB coating by means of pressure to cause the dyestuff precursor solution to be exuded onto the front of the mated sheet below it. The colorless or slightly colored dyestuff precursor then reacts with the color developer in the areas at which pressure was applied, thereby effecting the colored marking. Such mechanism for the technique of producing pressure-sensitive recording papers is well known.
Also well known are self-contained (SC) sheets which have the CB coating and the CF coating layered or admixed on a support sheet. Such sheets are also considered to be carbonless copy papers.
Microencapsulation has been used in the production of carbonless copy papers for some time. It is well known to use MF in the microencapsulation process as the material to construct the microcapsule wall.
Typically, a water soluble MF pre-condensate is dissolved in an aqueous solution (the external or continuous water phase). An oily solution containing therein a dissolved dyestuff precursor solution (the internal phase or core material) is dispersed in the water phase using a water soluble polymer as an emulsifier which can be either ionic or non-ionic in nature. A self-condensation reaction of the MF is initiated by lowering the pH of the emulsion and adding heat thereto. As the molecular weight of the MF pre-condensate increases, it precipitates (or more precisely, liquid-liquid phase separates) onto the oil droplets whereon further condensation of the MF completes the formation of the capsule wall.
Variations of this general reaction scheme can be found in the prior art. For example, U.S. Pat. Nos. 4,460,722 and 4,562,116 disclose the simultaneous condensation of a water soluble cationic urea resin along with a water soluble MF pre-condensate from an aqueous solution onto the surface of an oil droplet.
Disclosed in U.S. Pat. No. 4,409,156 is the addition of a water soluble styrene-sulfonic acid polymer to the water phase prior to initiation of the self-condensation reaction of a water soluble MF pre-condensate. Similarly, U.S. Pat. Nos. 4,406,816 and 4,574,110 disclose the inclusion, to the external phase prior to MF self-condensation, of a polymer having attached thereto a sulfonic acid group and an acrylic copolymer, respectively.
All traditional MF-type encapsulation methods found in the prior art utilize an aqueous external phase to serve as both the medium in which the microcapsules are formed and as the vehicle in which the microcapsules are coated as a part of the CB materials. Water, however, is unsuitable as a coating vehicle in certain applications. Following the application of the coating composition, the coating vehicle must be evaporated. In the case of water, this requires a considerable input of energy and the use of complex and expensive equipment. For a variety of reasons, such energy usage, complexity, and cost cannot be tolerated in some applications. Another problem of concern is the removal of the polluted water which emanates from the production and purification of the aqueous coating composition. Additionally, certain substrates such as paper are sensitive to water and tend to wrinkle upon drying.
In these types of applications where water cannot be used as the sole coating vehicle, an alternative must be used. The most common alternative to water as a microencapsulation medium and coating vehicle is an organic solvent. Organic solvents have inherent disadvantages as well (mainly in the recovery of evaporated organic solvent vapors), but in those applications where large amounts of water are unsuitable, these disadvantages are either avoidable (by using nonvolatile solvents) or are generally an acceptable compromise. Thus, it is desirable and even necessary in some applications to use organic solvents or water-organic solvent blends as the microencapsulation medium and coating vehicle
However, traditional MF microencapsulation techniques do not permit MF microencapsulation in an organic medium. To the extent that organic solvents are present in the external phase, the MF self-condensation reaction cannot take place. For this reason, all of the traditional MF microencapsulation methods found in the prior art must be conducted using water as a microencapsulation medium and, consequently, as a coating vehicle. Organic solvents that are used as a microencapsulation medium and coating vehicle contain chemical groups that react with the MF pre-condensate under acidic conditions. Such acidic conditions are required to initiate the MF self-condensation reaction. Alcohol, ester, amide, hemiacetal, and acetal groups commonly found in organic solvents used in coating formulations react with the MF pre-condensate to produce useless by-products.
At higher concentrations of organic solvent in the external phase, the competing MF-solvent side reactions dominate, and the MF self-condensation reaction occurs only to a minor extent (if at all). Only low molecular weight polymers and modified MF condensates that remain plastic are formed when self-condensation of MF is attempted in a medium containing a relatively high concentration of organic solvent. Such by-products do not separate from the external phase to collect on and coat the oil droplets. As a consequence, attempts to form MF microcapsules in an external phase containing a high concentration of organic solvent and using aqueous methods known in the prior art with water-soluble MF pre-condensates result in failure. Thus, MF microencapsulation techniques of the prior art cannot be utilized in applications where water is not suitable but where organic solvents would otherwise be suitable.
Accordingly, the need exists in the art for a MF microencapsulation process that can take place in an external phase containing high concentrations of organic solvents and where water alone is not suitable as a coating vehicle.